Surface Modification of Magnesium and its Alloys for Biomedical Applications

Modification and Coating Techniques

T.S.N. Sankara Narayanan,Il-Song Park,Min-Ho Lee

• Publisher: Woodhead Publishing
• Year: 2015
• ISBN:
  9781782420835
• Topic: Medizin
• Subtopic: Medizintechnik & Zubehör

The development of biodegradable implants which can remain in the human body to fix a problem and subsequently dissolve, or be absorbed, consumed or excreted, without warranting a secondary surgery, is very appealing to scientists. Due to their excellent biocompatibility and biodegradability, magnesium implants provide a viable option many problems associated with permanent metallic implants such as, restenosis, thrombosis, permanent physical irritation, and inability to adapt to growth and changes in human body. Volume 2 of this important new book explores practical issues of magnesium and magnesium alloys, physical and mechanical modification and coatings to enhance this material for biomedical applications.

• Includes expert analysis on chemical solution deposition of hydroxyapatite (HAp) and octacalcium (OCP) phosphate coatings for magnesium
• Comprehensive coverage of biomimetic modifications, surface functionalization of biomolecules, natural, conducting and biodegradable polymeric coatings
• Lucid dissection of chemical, physical, mechanical and electromechanical modifications of magnesium and its alloys for biomedical applications

Part One

Chemical and physical modifications of magnesium and its alloys for biomedical applications

Outline

1. Fluoride conversion coatings for magnesium and its alloys for the biological environment

2. Phosphate treatment of magnesium alloy implants for biomedical applications

3. Chemical solution deposition of hydroxyapatite and octacalcium phosphate coatings for magnesium and its alloys to improve biocompatibility

4. Physical vapour deposition on Mg alloys for biomedical applications

1

Fluoride conversion coatings for magnesium and its alloys for the biological environment

Thiago F. da Conceição¹, and Nico Scharnagl^{2 1}Departamento de Química - CFM, UFSC, Campus Trindade, Florianópolis/SC, Brazil ²Helmholtz-Zentrum Geesthacht Centre for Materials and Coastal Research, Geesthacht, Germany

Abstract

Studies in the literature have shown that magnesium fluoride is an interesting coating for protecting magnesium and magnesium alloys from corrosion in biological environments. In this chapter, this coating formation process and its relevant properties are discussed in detail. The corrosion performance of magnesium alloys with fluoride coating in regular saline solution, simulated body fluid, and in vivo is described. The current challenges and future trends are critically discussed based on recent reports in the literature.

Keywords

Corrosion; Fluoride conversion coating; Magnesium

1.1. Introduction

In the last few years, magnesium and its alloys have been considered as biodegradable materials for applications such as orthopedic implants and stents for vessel dilatation (Witte, 2010; Virtanen, 2011). Due to its biocompatibility (of magnesium and of its corrosion products) and good mechanical properties, magnesium-based implants may serve as load-bearing devices, contributing to the healing of the organism and then gradually degrading without causing adverse effects. Among the beneficial effects of such devices is that surgery for implant removal is not required. However, magnesium alloys have low corrosion resistance in aqueous environments containing ions such as Cl⁻, like the biological environment. Therefore, magnesium implants in such mediums may undergo earlier failure, excessive hydrogen production (which forms gas cavities and inflammation), and a high pH increase in the neighborhood of the implant, causing postoperatory complications (Virtanen, 2011).

The literature reports numerous surface treatments and coatings to control magnesium corrosion in biological environments (Hornberger, Virtanen, & Boccaccini, 2012). Among these, the preparation of fluoride conversion coatings has received special attention. The most studied fluoride conversion coating on magnesium is magnesium fluoride (MgF₂), which can be easily prepared by immersion of magnesium in a solution containing fluoride anions, as hydrofluoric acid (HF) solutions. Reports in the literature suggest that, besides increasing corrosion resistance, this conversion coating has antibacterial properties (Lellouche, Friedman, Lelouche, Gedanken, & Banin, 2012; Lellouche, Kahana, Elias, Gedanken, & Banin, 2009) and may induce bone healing due to beneficial effects of fluoride (Berglundh, Abrahamsson, Albouy, & Lindhe, 2007). In this chapter, the formation of MgF₂ coating on magnesium alloys is discussed. The potential of this treatment to protect magnesium from corrosion and to enhance its corrosion resistance in biological environments is considered in detail, as well as future trends and current challenges.

1.2. Coating formation: Mechanism and characteristics

1.2.1. Hydrofluoric acid immersion

The traditional method for preparing fluoride coating on magnesium and its alloys is to immerse the sample in an aqueous solution of hydrofluoric acid for a time period that can vary from a few minutes to days. It is generally assumed that the coating is formed by the reaction of magnesium with HF, as shown in Eqn (1.1). This reaction has a negative change in the Gibbs free energy, indicating that this is a product-favored reaction (the change in free energy was obtained using the data in chemical thermodynamic tables, in the temperature of 298.15 K, reported by Wagman et al., 1982). The literature reports different conditions for this treatment in regard to treatment time, acid concentration, and substrate pretreatment. Table 1.1 shows some characteristics of HF treatment described in the literature for different magnesium alloys. A systematic investigation on solution concentration and treatment time on the coating properties is reported in detail by Conceição, Scharnagl, Blawert, Dietzel, and Kainer (2010), Verdier, Laak, Delalande, Metson, and Dalard (2004), and Bakhsheshi-Rad, Idris, Kadir, and Daroonpavar (2013). Coating properties such as thickness, constitution, and porosity change significantly depending on these parameters.

Mg(s)+2HF(aq)→MgF2(s)+H2(g),ΔrG°=−476.6kJ/mol

(1.1)

In the studies of Conceição et al. (2010) and Verdier et al. (2004) it was shown that the coating can present a considerable amount of hydroxides and oxides, even when ground samples are used. In general, the lower the acid concentration the higher the amount of hydroxides/oxides formed on the metal surface. This observation is related to the possible formation of magnesium hydroxide during the HF treatment in aqueous solutions. Magnesium hydroxide can be formed by the reaction of the metal with water (Eqn (1.2)), which is the main reaction in the aqueous corrosion of magnesium. By comparing Eqns (1.1) and (1.2) it can be seen that both reactions have a similar thermodynamic tendency to take place. Therefore, from a thermodynamic point of view, it is expected that these reactions occur simultaneously when metallic magnesium is in contact with water and HF. The rate of each process will depend on the HF concentration.

Mg(s)+2H2O(l)→Mg(OH)2(s)+H2(g),ΔrG°=−359.3kJ/mol

(1.2)

Increasing the acid concentration assures a coating constituted basically...